When loading or unloading a truck parked at a loading dock, it is generally a safe practice to help restrain the truck from accidentally moving too far away from the dock. This is often accomplished by a vehicle restraint that engages what is referred to, in the industry, as a truck's ICC bar (Interstate Commerce Commission bar) or RIG (Rear Impact Guard). An ICC bar or RIG is a bar or beam that extends horizontally across the rear of a truck, below the truck bed. Its primary purpose is to help prevent an automobile from under-riding the truck in a rear-end collision. A RIG, however, also provides a convenient structure for a vehicle restraint to engage, thereby obstructing the bar's (and thus, the truck's) movement away from the dock. To release the truck, at least a portion of the restraint is lowered to a stored position below the bar, which also allows the next truck to back into the dock.
There are at least two general types of RIG-engaging vehicle restraints. A first type of RIG-engaging vehicle restraint relies on the power of the truck backing into the dock as the impetus for operating the vehicle restraint. This type of vehicle restraint may use spring force for storing the restraint in a normally raised position. As a truck backs its RIG over the upwardly biased vehicle restraint, the RIG engages a ramp or some other type of mechanical actuator that forces the restraint down, underneath the RIG. When the truck's RIG is properly positioned over the restraint, a relatively small power unit can be actuated to raise a barrier portion of the restraint in front of the RIG. Examples of such truck-powered vehicle restraints that store in a normally raised position are disclosed in U.S. Pat. Nos. 6,190,109; 6,322,310; 5,882,167; 5,702,223 and 5,297,921, all of which are specifically incorporated by reference herein.
In addition to utilizing the truck's power to operate the vehicle restraint, the spring of such restraints also enables upwardly biased restraints to follow the incidental vertical movement of the RIG as the truck is being loaded or unloaded of its cargo. This can be advantageous in comparison to other types of vehicle restraints.
A second general type of vehicle restraint stores in a lowered position and typically requires some type of power unit, such as a motor or hydraulic cylinder, to raise the restraint to an elevation where it can capture the RIG. Since the power unit must raise the entire moving portion of the vehicle restraint, lifting such weight may require a power unit of substantial size and horsepower, which can add even more weight to the restraint. The power unit of this vertically moving restraint may include a small spring, or other mechanism, to accommodate slight vertical movement of the truck/RIG, but a vehicle restraint of this type typically has no mechanism for accommodating horizontal movement of the RIG.
A limitation common to both types of restraint is an inability of the restraint to follow the horizontal movement of the RIG. For example, after a vehicle restraint is initially positioned relative to the RIG, the loading or unloading operation may cause the truck to move both vertically and horizontally. The vehicle restraint's barrier prevents the RIG from moving forward horizontally, in a direction opposite the dock face, but it does not generally accommodate the RIG moving backward in a horizontal direction toward the dock face (away from the restraint's raised barrier). This can leave a horizontal gap between the RIG and the barrier even though the spring or actuator allows for vertical movement and continues to hold the restraint tightly up against the underside of the RIG. If the vehicle later begins to return to its more forward position, the gap provides a backlash in which the RIG can accelerate before striking the barrier with an impact that may be sufficient to bend or otherwise damage the RIG. In the case of a premature truck/trailer departure, this horizontal gap may actually allow a trailer to build up enough momentum to “jump” over the barrier prior to the operator lowering the restraint and releasing the trailer.
Known prior art restraints included no means for detecting the horizontal position of the RIG relative to the vehicle restraint's barrier. Because the RIG's horizontal position relative to the barrier was not detected, prior art restraints did nothing to eliminate the potentially hazardous horizontal gap that may result from the backward movement of the trailer (and RIG), nor did they do anything to alert workers of such a hazardous condition. Consequently, a need exists for a vehicle restraint that can properly respond not only to vertical movement of a RIG but also to horizontal movement of a RIG to warn of, and preferably minimize, a gap that may form between the barrier and the RIG upon horizontal movement of the RIG.